The present invention relates to a method for processing ultrasonic echo signals of both directionally reflecting and non-directionally scattering objects, particularly for ultrasonic image processing in the field of substance or tissue investigation in which ultrasonic echo signals proceeding from various scanning directions are combined.
Known methods for processing ultrasonic echo signals cannot process signals proceeding from exactly reflecting and isotropically scattering object ranges with equal quality. Areas with a highly directional reflection, such as, for example, organ contours or cranium bones of an embryo, are imaged well in the known "B-scan method" insofar as they are oriented in such manner that the reflected echoes strike the receiver. In contrast thereto, centers which scatter isotropically are only weakly reproduced and blurred in the "B-scan method" and can therefore be covered in the image by other image elements.
The B-scan image of ultrasonic imaging known from previously known employments has further deficiencies in addition to the above disadvantages. Thus, among other things, the resolution of such ultrasonic images at right angles to the irradiation direction is significantly poorer than in the irradiation direction. Moreover, signals from individual scattering centers are suppressed in comparison to the stronger reflection signals of surfaces.
These deficiencies are partially overcome in that B-scan images from a plurality of directions are superimposed according to the known compound-scan method. Therewith, reflecting surfaces can be generally imaged rather well. The resolution given punctiform scattering centers, however, is worsened. Due to the blurring of the point image at right angles to the direction of irradiation and due to the super-imposition, punctiform scattering centers are blurred in a star shape.
In the known method of "X-ray computer tomography",every point is expanded or spread into a strip. The strips gained in such manner are filtered and superimposed in accord with a method designated below as "spread image method".
In contrast thereto, this spread image method of X-ray computer tomography reproduces object parts with a locally isotropic scatter behavior very well. Significant image errors, however, occur given greatly anisotropic, i.e., exactly reflecting surfaces.
The attempt to transfer the spread image method to objects with precisely reflecting boundaries must fail. This becomes understandable when one considers that, given this method, strips with respectively equal echo transit times are superimposed from different irradiation directions. An isotropically scattering center contributes with equal strength to each strip proceeding through it and is therefore reproduced at its location in the superimposition. A point with a precisely reflecting environment, however, contributes to only one strip. This is orthogonal to the respective reflection direction. It contributes nothing to the other strips passing through said point. In the superimposition, therefore, it is practically suppressed. Standard convolution for filtering of the image gained from the superimposition of said strips has an additional negative influence on the result in the case of exact reflection. This convolution is only correct given "isotropic" superimposition. Missing image amounts give rise to disruptions.
Extensive investigations in the area of ultrasonic echo image generation concerning the reflection and scatter behavior in biological agents have shown that both isotropic scatter behavior and exact reflection and their mixed forms play a significant role.